The invention relates to a method of manufacturing a hot-cathode element from a rare-earth-oxide-doped refractory metal where initially a plurality of sub-layers of the hot-cathode element are successively deposited on a substrate member by means of a CVD process. The invention also relates to a cathode ray tube comprising such a hot-cathode element.
A method of this kind is described in EP-A-0204 356. Therein, during a plasma-activated CVD coating (PCVD) of an inner surface of a substrate cylinder, tungsten carbide (W.sub.2 C) is also introduced into a hot-cathode element consisting of thoriated tungsten. Because of the C-content, during and after the later activation of the hot-cathode element during operation, the formation and sustenance of an approximately monatomic film of thorium on the surface of the hot-cathode element at comparatively low temperatures of approximately 2000K is stimulated. The W.sub.2 C which is important for the operation of a thoriated tungsten cathode ensures that metallic thorium is released via the reaction ThO.sub.2 +2W.sub.2 C.fwdarw.Th+4W+2CO.uparw. and is diffused along the grain boundaries to the surface where it supplements the essentially monatomic Th film deposited at that area. On the other hand, the carbon content causes embrittlement of the hot-cathode element, so that it must be very carefully handled in order to avoid fractures.